1. Field of the Invention
This invention relates to a water-insoluble hydrogel, a method for the production thereof, and an ultrasonic transmitting medium. More particularly, it relates to a water-insoluble hydrogel having a high water content, permitting easy sterilization, excelling in biocompability, exhibiting high mechanical strength, and producing pleasant tactile sensation, a method for the production thereof, and an ultrasonic transmitting medium using the water-insoluble hydrogel.
2. Description of the Prior Art
For the cardiotomy to be performed precisely on the ailing heart, it is desirable that the patient should be given a deliberate diagnosis prior to the operation for the determination of the position and shape of the morbid region, the relation of the morbid region with the adjoining normal region, and so on. Particularly, in a serious case involving a neonate, this diagnosis constitutes itself an important task which determines the outcome of the operation. The ultrasonic diagnosis has advanced and disseminated remarkably in recent years as a non-invasive testing method applicable to the circulatory region. The accuracy of the preoperative ultrasonic diagnosis is contributing immensely to the improvement of results of recent surgical operations. By the preoperative sonic test carried out by applying a probe on the wall of the chest while avoiding the lungs and the bones, no sufficient information is obtained concerning the cardiac disease and the morbid location. As a way of conducting a more deliberate ultrasonic diagnosis beyond the existing limit, the act of carrying out the ultrasonic test during the course of cardiotomy is conceivable. To be more specific, when the ultrasonic test is carried out after the sternum has been incised mediated and before the pericardium, the heart, or the blood vessel is incised by directly applying the probe on the outside of the organ, a more detailed surgically useful diagnosis is obtained with respect to the lumen of the organ. By this method, it is made possible to confirm during the course of a surgical operation whether or not the surgical treatment being carried out in consequence of the ultrasonic test made during the course of surgical operation is proper. This fact is important for the prevention of repetition of a surgical operation and the postoperative management of the patient.
When the ultrasonic test is carried out by directly applying the probe on the wall of the heart or the blood vessel, the heart or the blood vessel is ununiformly moving in concert with the heartbeat and the pressure of the probe tends to induce arrhythmia or lower the blood pressure. It is, therefore, difficult for the conventional probe possessing an inflexible contact surface to be safely used as incessantly held tightly on the surface of the pulsating heart. In actuality, the usefulness of the real-time ultrasonic laminagraphic testing method performed during the course of a surgical operation is not fully manifested. A suitable substance is required as a contact medium for use in the ultrasonic test during the course of a surgical operation. Specifically, a need is felt for a gel-like elastic substance possessing softness and strength necessary in following the movement of the heart while adhering tightly to the surface and, at the same time, excelling in biocompability and ultrasonic characteristic.
Heretofore, as a material for use in medical devices and implements, a hydrogel having not less than 40% of water contained in a structure produced by occluding polyvinyl pyrrolidone in a cross-linked polyvinyl alcohol type hydrophilic macromolecular network has been known. This hydrogel is obtained by mixing a polyvinyl alcohol type hydrophilic macromolecular substance with polyvinyl pyrrolidone and thereafter insolubilizing only the polyvinyl alcohol type hydrophilic macromolecular moiety of the mixture (Japanese Patent Publication SHO 58(1983)-44,699).
This hydrogel, however, poses as a problem the treatment of the unaltered cross-linking agent such as concentrated sulfuric acid and, moreover, has the disadvantage that it is not easily sterilized, though it excels in biocompability. For example, it offers no sufficient resistance to heat when it is sterilized in an autoclave. When it is sterilized with ethylene oxide gas, since the hydrogel occludes a large amount of water, the ethylene oxide gas is absorbed in the water and suffered to remain therein or part of the absorbed ethylene oxide gas is converted into ethylene glycol and again suffered to remain as such. Owing to the toxicity of the persisting ethylene oxide or ethylene glycol, the hydrogel is not allowed to be sterilized with the ethylene oxide gas. When the hydrogel is sterilized by radiation, the cross-linking proceeds to excess and the reaction of a free radical present in the material gives rise to oxygen, hydrogen peroxide, hydrogen, etc. Thus, the hydrogel which has undergone this sterilization is no longer suitable for the intended use.
As materials for use in such medical devices and implements, we have proposed water-insoluble hydrogels which contain 50 to 99% by weight of water and which are produced by cross-linking two polyvinyl alcohols, two polyvinyl pyrrolidones, and a polyvinyl alcohol and a polyvinyl pyrrolidone by exposure to radiation with a polymerization degree enough for the occlusion of the amount of water mentioned above (Japanese Patent Application SHO 62(1987)-128,475). This water-insoluble hydrogel has a high water content, allows easy sterilization, and excels not only in biocompability but also in acoustic characteristic and, therefore, proves to be useful for such medical implements as ultrasonic transmission medium. It nevertheless has the problem that it has no sufficient physical strength, peels off the skin on exposure to the impact of touch with the clothing, and sustains breakage while in use. Immediately after removal from a wrapper, it is slippery to the touch. The hands used in handling it must be given thorough washing. Thus, a desire has been expressed to improve this hydrogel. It is possible, for example, to enhance the physical strength by increasing the dosage of radiation thereby heightening the cross-link density. This measure, however, degrades the elasticity of the hydrogel and imparts brittleness to it and affects the occurrence of bubbles during the course of reaction. Thus, it can be hardly called a suitable method.
An object of this invention is to provide an improved water-insoluble hydrogel and a method for the production thereof.
Another object of this invention is to provide a water-insoluble hydrogel having a high water content, allowing easy sterilization, excelling in biocompability, exhibiting high physical strength, and producing pleasant tactile sensation and a method for the production thereof.
A further object of this invention is to provide an ultrasonic transmission medium using the hydrogel mentioned above.